Polygonal-shaped radiation detector employing plural prism-shaped semiconductor crystals

ABSTRACT

The invention relates to a semiconductor detector for measuring and/or detecting ionizing radiation, having a large sensitive volume, particularly for gamma spectrometry.

United States Patent [7 2] Inventors Jean Antoine Cacheux;

Johannes Meuleman, both of Caen, France [21] App]. No. 800,405

[22] Filed Feb. 19, 1969 [45] Patented Nov. 16, 1971 [7 3] Assignee U.S. Philips Corporation New York, N.Y.

[32] Priority Feb. 19, 196B [33] France [54] POLYGONAL-SHAPED RADIATIONDETECTOR EMPLOYING PLURAL PRISM-SHAPED SEMICONDUCTOR CRYSTALS 6 Claims,4 Drawing Figs.

[52] US. Cl

[51] int. Cl [50] Field of Search [56] References Cltedl UNITED STATESPATENTS 3,233,102 2/l966 Packard 3,293,435 l2/l966 Huth PrimaryExaminer-James W. Lawrence Assistant Examiner-Davis L. Willis Attorney-Frank R. Trifari ABSTRACT: The invention relates to a semiconductordetector for measuring and/or detecting ionizing radiation, having alarge sensitive volume, particularly for gamma spectrometry.

POLYGONAlL-SHAPED RADlATllON DETECTOR EMPLOYHNG PLURAL PRISM-SHAPEDSEMHCONIDUCTOR CRYSTALS It is known that the possibilities of use ofradiation detectors can be extended by enlarging the volume of thesensitive portion of the detectors. By enlarging the surface of theplane of incidence and of the useful section of the semiconductorcrystal the quantity of incident radiation received may be increased,while by enlarging the thickness of the sensitive portion the radiationabsorption can be increased, particularly when the radiation has a greatpenetration depth.

In the gamma spectrometry there are known lithiumcompensated germaniumdetectors having flat junctions and thick sensitive zones. It is knownthat lithium is an interstitional donor element having a very highdiffusion coefficient and when incorporated as an impurity in asemiconductor crystal of P-type conductivity it tends to neutralize theeffect of the acceptor atoms, for example, of boron, in the crystal.Appropriate choice of the concentrations of the P-type impurities and oflithium permits of obtaining a compensated semiconductor zone having apractically intrinsic conductivity and a high resistivity, which mayform a thick sensitive zone for the detection of ionizing radiation. Thethickness of such a compensated sensitive zone, which may be obtained bythe supply of lithium ions from a surface in the crystal under theaction of an electric field, is, however, restricted. Thickercompensated zones have been obtained by diffusing lithium into thecrystal simultaneously from two opposite faces ofa crystal. By knowntechniques sensitive zones may be obtained whose thickness is restrictedto about 20 mms at the most, while the sensitive volume is about ccms atthe most.

There are furthermore known so-called coaxial detectors having a largesensitive volume, in which the.crystal has the shape of a cylinder or ofa prism having a trapezoidal base. The latter shape matches that ofsingle crystals obtained by crystallization in a horizontal boat. Thesecoaxial detectors have a junction of substantially cylindrical shape,the axis of which is parallel to the generatrices of the detector.

The volume of the sensitive portion of this coaxial detector is limitedby the volume of the obtainable single crystals and it should be notedthat, as is known the probability of crystal defects rapidly increaseswith the dimensions of the single crystals. Although detectors of thiskind of may be made with a volume of the order of for example 50 ccmspulling of the single crystals involves too many problems for industrialmanufacture. Moreover, the depth of the lithium-compensated zone whichis obtainable as a maximum, and which is formed by diffusion from theouter surface of the cylinder and/or from the surface of a cavitythereof, restricts the dimensions of the detector, while this involvescomplications of the processes.

Moreover, in these cylindrical or prismatic detectors the distributionof the electric field is not as uniform as in a detector having a flatjunction. The collecting times of the charges and the rise times of thepulses are variable and the resolving power is lower. lN addition, thecapacitance of these detectors is very high.

in addition, in order to facilitate transport and storage and tomaintain the quality of the detector the detector is preferably to beprovided with a closed envelope protecting it from soiling. This is themore required for lithium-compensated germanium detectors because areemployed at a temperature comparable with that of liquid nitrogen andhave to be stored at low temperatures.

in order to obtain a high detection efficiency and a high resolvingpower it is advantageous to have a detector oflarge sensitive volumeavailable which has one or more flat junctions bounding a uniform field,while the single crystal has minimum dimensions and is provided with aneffective protection.

The invention has for its object to provide a detector which satisfiesthese conditions and is based on the recognition of the fact that thiscan be achieved by composing the detector of a plurality of detectionelements having flat junctions, the shape of the detection elementsbeing advantageously chosen so that a detector of coaxial structure canbe obtained, in which on the one hand the advantages of detectors havingflat junctions are combined with those of coaxial, large-volumedetectors and on the other hand the requirements for protection and theaforesaid conditions of use are satisfied.

According to the invention, a semiconductor detector of the kind setforth is characterized in that it comprises a plurality of semiconductordetection elements whose semiconductor crystals have the shape of aprism having an equal-sided trapezoidal base and at least onesemiconductor junction extending approximately parallel to the twoparallel side faces of the crystal, said detection elements beingarranged in a regular polygon so that the smaller of said parallel sidefaces of each crystal is orientated towards the center of the polygon.

Consequently, the detector according to the invention has the structureof a coaxial detector. The useful sensitive zones form a regularpolygonal ring, while each sensitive zone and each semiconductorjunction has a flat structure.

in a preferred embodiment of the invention each detection elementcomprises a semiconductor germanium crystal whose portions adjacent ofthe two parallel side faces are of the one conductivity type, whereasbetween these portions two practically completely compensated zones arelocated which are separated from each other by a stratified zone of theother conductivity type.

The two portions of the one conductivity type are preferably doped withlithium, the two zones being compensated by lithium diffusion under theaction of an applied electric field. The shape of each element matchesthat of single crystals as obtained by known techniques by horizontalcrystallization and therefore these crystals can be used substantiallywithout loss of material.

All detection elements are identical so that they can be exchanged andeach defective element can be simply replaced so that one defect doesnot render the whole detector unserviceable.

As compared with the conventional coaxial detector having the samesensitive volume, the detector according to the invention has greaterreliability, since the quality of the separate small single crystalswill usually be superior to that of one large single crystal of theknown detector. X

The detection elements of the detector according to the invention may bejoined inside a common, closed envelope or be arranged without anenvelope in apparatus such as cryostats, vacuum spaces and so on.

Each detection element is preferably provided with a closed envelopehaving the shape of a prism surrounding the single crystal and having atrapezoidal base.

With lithium-compensated germanium detectors it is often necessary tocarry out a redistribution of the lithium ions during thelastmanufacturing stage or during use in order to restore thecompensation in the sensitive zone. The construction of the detectoraccording to the invention in which the detection elements are arrangedseparately in a prismatic envelope having a trapezoidal base rendersthis redistribution feasible in a simple manner.

The annular structure of the detector according to the invention has afairly large free space in the axis of the detector so that it may beemployed as a detector of the well" -type for analyzing a samplearranged in the center ofthe ring.

The number of detection elements of the detector according to theinvention is not restricted. In; an advantageous em bodiment this numberis 3 to l2 and the angle between the nonparallel side faces of eachenvelope or of each crystal is then equal to the quotient of 360 andsaid number. This angle is equal to 45 in the case ofeight elements.

Each detection element itself is a detector having a flat junction and auniform field and a symmetrical structure which permits of obtainingvery high resolving powers supplying pulses of short rise time, becausethe collecting times of the charge carriers are very short.

An advantage of the multiple structure of the detector according to theinvention is furthermore that the detection elements may separately beassociated with an electronic amplifying device or at least preamplifierso that high counting frequencies can be obtained.

The structure of each detection element is furthermore such that animproved detection factor is obtained by the reduction of the Comptoneffect. If radiation penetrates through the smaller of the two parallelside faces the trapezoidal shape provides an improved ratio between thepeak value due to the overall energy and the peak value due to theCompton effect. If opposite elements of a detector are connected toseparate preamplifiers in coincidence a separation can be obtainedbetween the signal produced by the photoelectric effect and the signalproduced by the Compton effect.

The dimensions of each detection element and those of the detectoraccording to the invention may be adapted to the desired use. For gammaspectrometry of radiation of about 2MeV the larger of the two parallelside faces of each element is preferably approximately square.

It is advantageous for each detection element to have a germaniumcrystal crystallized along the (11 1) plane, having lithium-compensatedzones and arranged in a closed envelope, preferably of aluminum, whichenvelope forms one of the electrodes of the detection element. The twoother electrodes may be taken through the wall by an insulated passageand inside the envelope a lower gas pressure or vacuum may be used.

Such envelopes can be manufactured in a conventional manner. Eachenvelope may be closed by cold-welding at the bottom or upper face ofthe prism.

The various elements of a detector according to the invention may, ofcourse, also be used separately or be joined to form part of a ring.

In some cases it may be advantageous to increase further the volume ofthe detector by arranging a coaxial detector in the central, free space,for use together with the annular elements.

The invention will be described more fully with reference to thedrawing, in which FIG. 1 is a diagrammatic perspective view of adetection element in accordance with the invention,

FIG. 2 is a diagrammatic sectional view of a semiconductor detector inaccordance with the invention, comprising a plurality of detectionelements of the type shown in FIG. 1,

FIG. 3 is a schematic side elevation of a detection element having aclosed envelope and FIG. 4 is a schematic sectional view taken on theplane IV-3 IV in FIG. 3.

The detection element shown in FIG. 1 comprises a single crystal 1having the form of a prism with an isosceles, trapezoidal base. Thiselement thus has two parallel side faces 2 and 3 and two oblique sidefaces 4 and 5. The crystal 1 may have a central P-type zone 6, to whichon both sides are joined a thick, practically intrinsic zone 7, 8respectively and an N-type zone 9 and 10 respectively. These zones arebounded by flat faces extending substantially parallel to the faces 2and 3 and forming semiconductor junctions.

The practically intrinsic zones may be formed in a conven tional mannerby diffusion of lithium ions into the semiconductor crystal under theaction of an electric field from the two faces 2 and 3, which crystalmay be of P-type germanium.

The detector shown schematically in FIG. 2 comprises eight prismaticelements 11 to 28 having isosceles, trapezoidal bases. These elements 11to 18 may be of the kine of shown in FIG. I. They may be directly joinedor be provided separately with a prismatic, closed envelope. Eachprismatic element 11 to 18 is arranged so that its oblique side face 19(corresponding with the face 4 of the crystal 1) is orientated to theside face 20 (corresponding with the face 5 of crystal 1) or a furtheradjacent crystal and the two side faces 21 (corresponding with face 3 ofcrystal 1) of two adjacent elements 11 to [8 are oriented to the sameside so that the elements together fon'n a regular polygon, in which thesmaller ones (21) of the parallel side faces of the detection elementsare orientated towards the center of the polygon.

Like in FIG. 1 the P-type zones 6, the practically intrinsic zones 7 and8 and the N-type. zones 9 and 10 are indicated by broken lines. Thesezones form together concentric, substantially annular sets.

In FIG. 2 the arrangement holding the elements 11 to 18 together andproviding the rigidity of the assembly is not shown for the sake ofclarity. A simple metal or, if necessary, insulating strap will sufficeand as a matter of course the device in which the detector is employed,for example, the cryostat to which or in which it is fastened plays apart.

FIGS. 3 and 4 show a semiconductor crystal in a separate prismaticenvelope.

The single crystal 1 is held in place in an envelope 22 by means ofblocks 23 to 27 of insulating material, forexample,polytetrafluoroethylene. The blocks 27 are held in position by ridges inthe wall of the envelope and at the position of the P- type zone 6 theybear on the side faces 4 and 5 of the single crystal. The blocks 27 maybe provided with a metallic layer, preferably aluminum, so that they mayestablish an electric connection between the zones 6 and the envelope.

The envelope 22 is provided with a lid 29, fastened by a cold weld 31 tothe collar 30 of the envelope. Because this connection, which has to beairtight, has to resist the temperature differences to which thedetector is exposed, inter alia when it is cooled to the temperature ofliquid nitrogen, the lid is provided with a deep groove 32 for avoidingundesirable stress.

The connections to the N-type zones 9 and 10 established via twopassages 38 and 50.

SAid passages are airtight. They are formed, for example, by a metaltube 40, preferably of copper, soldered in a ferronickel sleeve 41 andinsulated from a ferronickel ring 42 by an insulating ring 43. The ring42 is soldered to a punched copper part 44, which is fastened by a coldweld 45 on the envelope usually of aluminum.

One of the tubes 40 is provided by welding with an elastic tag. This tagestablishes a pressure contact with the zone 9. The other tube hasfastened to it an insulated conductor (not shown in FIG. 4, whichestablishes the contact with the zone 10.

Inside the envelope, in the space 47 between the envelope and thecrystal 1, vacuum prevails which is obtained through the tubes 40, whichcan be closed subsequently by compression. The envelope may, as analternative, contain an inert .gas under low pressure.

The envelope described above is preferably made of aluminum of about 0.5mm. in thickness.

The connections of the various regions of a single crystal may differfrom those shown in FIGS. 3 and 4. For example, the lid 29 at the bottomof the envelope 49 may be provided with an insulated passage similar tothe passage 50 or of a different type. In some cases a surface zone, forexample, the zone 10 or 9 (FIG. 4) may be connected with the envelope bydirect pressure contact.

In the case of envelopes of the kind shown in FIGS. 3 and 4 thedetection elements may be arranged in order of succession in onedirection and in the opposite direction so that the side faces 19 and 20of two consecutive envelopes may be opposite each other without the lidsand the flanges 30 of two adjacent envelopes hindering each other.

The embodiment shown in FIG. 2 comprises eight elements and isparticularly advantageous in the case of single crystals in which thesides of the trapezoidal section have a symmetric slope of 33 30' K. Itis, of course advantageous to adapt the section of the semiconductorcrystal of an element to the number of elements of the detector orconversely.

It will be obvious that the invention is not restricted to theembodiments described above and many variants are possible to thoseskilled in the art within the scope of this invention. For example, thesemiconductor crystals may consist of silicon or course, A'-B" compound,while the crystals may have a single compensated zone instead of two.Compensation with lithium may be completely avoided by using as astarting material for the detection elements semiconductor crystals ofhigh resistivity, which may be provided with diffused zones of oppositeconductivity types.

in the claims:

l. A semiconductor detector for ionizing radiation comprising aplurality of semiconductor detection elements forming a polygon, each ofsaid elements comprising a semiconductor prism-shaped crystal having anisosceles trapezoidal base with parallel side faces of unequal size andoblique side faces, each of said crystals having at least onesemiconductor electrical junction extending substantially parallel tothe parallel side faces, said detection elements being arranged to forma regular polygon with the smaller of the parallel side faces of eachcrystal oriented towards the polygon center and with the oblique sidefaces adjacent one another.

2. A semiconductor detector as claimed in claim ll wherein each oi thedetection elements is provided with an envelope also in the form of aprism having an isosceles trapezoidal base.

3. A semiconductor detector as claimed in claim 11 wherein thesemiconductor crystal of each of the detection elements is a germaniumcrystal whose portions adjacent the two parallel side faces are of theone conductivity type, between which portions are provided twopractically compensated zones separated from each other by a layer olthe other conductivity type, said zones and portions forming fiveconsecutive zones,

4. A semiconductor detector as claimed in claim 1 wherein the number ofdetection elements is at least 3 and at the most 112.

5. A semiconductor detector as claimed in claim 11 wherein the larger oithe two parallel side faces of each detection element is substantiallysquare.

s. A semiconductor detector as claimed in claim 2 wherein the envelopeis substantially completely of aluminum and constitutes one of theelectrodes of the detection element.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent 3621256Dated November 16, 1971 JEAN ANTOINE CACHEUX and JOHANNES It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

lIn the Abstract, the abstract should read as follows: I

-- A semiconductor detector, especially for gamma radiation, having alarge sensitive volume is described. This is obtained by usingsemiconductor crystals which are prism-shaped with a trapezoidal basearranged in a circle to form a regular polygon defining a coaxialdetector. Lithium compensated germanium may be employed for thesemiconductor elements.

Col. 1, between the title and the first paragraph, the followingparagraph should be inserted:

The invention relates to a semiconductor detector for measuring and/ordetecting ionizing radiation, having a large sensitive volume,particularly for gamma spectrometry.

line 42, "X" should be deleted.

line 49, "IV-3 IV" should read IV-IV Col. 3, line 65, "kine of" shouldread kind Col. line 75, course" should read an Signed and sealed this30th day of M 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JB. ROBERT GOTTSC Attesting Officer Comissioner i'atents

2. A semiconductor detector as claimed in claim 1 wherein each of thedetection elements is provided with an envelope also in the form of aprism having an isosceles trapezoidal base.
 3. A semiconductor detectoras claimed in claim 1 wherein the semiconductor crystal of each of thedetection elements is a germanium crystal whose portions adjacent thetwo parallel side faces are of the one conductivity type, between whichportions are provided two practically compensated zones separated fromeach other by a layer of the other conductivity type, said zones andportions forming five consecutive zones.
 4. A semiconductor detector asclaimed in claim 1 wherein the number of detection elements is at least3 and at the most
 12. 5. A semiconductor detector as claimed in claim 1wherein the larger of the two parallel side faces of each detectionelement is substantially square.
 6. A semiconductor detector as claimedin claim 2 wherein the envelope is substantially completely of aluminumand constitutes one of the electrodes of the detection element.